Abstract: A positive association between World Trade Center (WTC)-exposure and thyroid cancer at a magnitude of at least double that of the general population has been found in four previous studies. WTC exposures from the dust that resulted from the collapse of the buildings, as well as from the fires that burned for months, contained some known carcinogens but to date, none have been definitively proven to be associated with thyroid cancer in non-WTC exposures. The most widely recognized causal factor for thyroid cancer is exposure to radiation; but only normally-occurring baseline levels were reported at the WTC site after the disaster. Nonetheless, the elevated risk of thyroid cancer might be due to WTC-related exposure or it might be due to other factors experienced by the WTC-exposed cohorts, such as incidental detection as a result of increased medical surveillance. A preliminary chart review among Fire Department of the City of New York (FDNY) WTC-exposed firefighters suggests incidental detection might be a factor in the diagnosis of thyroid cancer. Through a collaborative effort between FDNY, the General Responder Cohort (GRC), WTC Health Registry (the Registry) and the Rochester Epidemiology Project (REP), we propose to quantify the extent to which the observed association between WTC exposure and thyroid cancer is influenced by increased medical surveillance among WTC-exposed rescue/recovery workers. The specific goals of this two-year project are: 1. To conduct a chart review and study how cases detected incidentally compare to symptomatic cases in terms of histological make up, cancer stage and mortality. 2. To compare the risk of thyroid cancer by detection method (incidental/symptomatic), among FDNY workers to the risk among Olmsted County residents from REP. 3. To compare the risk of thyroid cancer among rescue/recovery workers in a medical monitoring program, the WTC Health Program (FDNY and GRC), to the risk among WTC-exposed rescue/recovery workers from the Registry in neither WTC Health Program. After comparing the overall risk, we will assess if the excess risk of thyroid cancer diagnosis among workers in the WTC Health Program compared to the Registry is limited to those who received CT scans as part of the increased medical surveillance in WTC Health Program cohorts and therefore due to incidental detection.